1. Field of the Invention
The present invention relates to an omniazimuthal visual system for use in a visual sensor, such as surveillance cameras, etc., which has a wide angle of view that enables omniazimuthal observation.
2. Description of the Related Art
In recent years, in the field of visual sensors such as surveillance cameras, etc., in various practical applications, a combination of a camera and a computer executes tasks which would have been carried out by a human with his/her own eyes in conventional applications. In general, a camera used in such applications has a limited angle of view. In some applications, a wide-angle lens (e.g., fisheye lens) is used for the purpose of obtaining a wider angle of view. Furthermore, in the field of mobile robots, studies for practical use of a mirror having a shape of a surface of revolution, such as a conical mirror, a spherical mirror, a hyperboloidal mirror, etc., are energetically carried out for the purpose of obtaining a wide angle of view.
In conventional visual sensors, an omniazimuthal image obtained by an optical system and an imaging device is processed by software used by a computer so as to be transformed into an image which is easier for a human to see, such as a panoramic image or the like.
FIG. 10 shows a conventional omniazimuthal visual system 1000. The omniazimuthal visual system 1000 includes: an optical system 1001 which utilizes a wide angle lens (e.g., fisheye lens) or a mirror having a shape of a surface of revolution (a conical mirror, a spherical mirror, a hyperboloidal mirror, etc.); an imaging section 1002 for converting an optical image obtained by the optical system 1001 into image data; and a computer (workstation) 1007 including an image converter which performs software processing, a display, and a display controller. In this omniazimuthal visual system 1000, a round-shape optical image obtained by the optical system 1001 is converted by the imaging section 1002 into image data of the round-shape image. This image data is supplied to the computer 1007, and processed by the computer 1007 using software into an image which is easier for a human to see, such as a square panoramic image or a perspective image.
The image transformation processing, through which a round-shape optical image is transformed into an image which is easier for a human to see, such as a square panoramic image or a perspective image, requires a number of coordinate transformations by which a polar coordinate is transformed with a trigonometric function into a rectangular coordinate. Furthermore, the coordinate transformation requires high accuracy.
Accordingly, in the conventional omniazimuthal visual system 1000, when the image transformation processing is carried out by software used by the computer 1007, 20 or more steps are required in arithmetic processing for a single data transformation even in the case of using a hyperboloidal mirror. Therefore, the image transformation processing which uses software requires a long processing time.
Among various mirrors having a shape of a surface of revolution, a hyperboloidal mirror is an optical element which enables a precise central projection transformation. With the hyperboloidal mirror, an image transformation can be carried out only with linear operations. On the other hand, in the case of using a wide-angle lens (e.g., fisheye lens) or another type of mirror having a shape of a surface of revolution, such as a conical mirror or a spherical mirror, an image transformation requires nonlinear operations in addition to trigonometric functions. Thus, in the case of using the hyperboloidal mirror, a processing speed is relatively increased as compared with an optical system using a wide angle lens (e.g., fisheye lens) or another type of mirror having a shape of a surface of revolution, such as a conical mirror or a spherical mirror.
Furthermore, when software is used in the image transformation, even in the case where a result of a constant operation is previously stored in a register, it is necessary to frequently repeat a step of reading out this result from a register, when it is required, and temporarily storing a calculation result again in the register. Therefore, a data processing time is determined according to the number of the repetitions of the step.
Although the processing time required for software processing of an image formed by a small number pixels might be short, a considerably longer processing time is required for software processing of an image formed by a greater number of pixels. Thus, for an image formed by 100,000 pixels, the maximum processing speed of the image transformation is several frames per second.
In the case of a still image, a long software processing time does not matter greatly. Therefore, a high resolution picture can be obtained even with a conventional omniazimuthal visual system. However, in the case of a dynamic image, along with an increased number of pixels, the processing speed of the software processing decreases so that the software processing cannot catch up with a required processing speed for dynamic images. As a result, the quality of the images obtained is significantly deteriorated. Thus, higher processing speed is required for processing of a dynamic image.